Thymus derived (T) cells were purified from normal human peripheral blood. Human T cells form rosettes with sheep red blood cells. After rosette formation and removal of rosette forming T cells from other cells, human T cells were found to respond to phytomitogens with less DNA synthesis than equal numbers of autologous mononuclear cells. This observation was not due to alterations in the T cells nor were they an artifact of the culture condition. The diminished mitogen induced activation of human T cells could be potentiated with autologous and allogeneic monocytes. Preliminary data have shown that activated T cells can mediate antibody dependent cellular cytotoxicity compared to inactivated T cells. Also, monocytes can restore the ability of x-irradiated (1000r) human T cells to respond to phytomitogens. BIBLIOGRAPHIC REFERENCES: J.R. Schmidtke and S.H. Hatfield. Activation of purified human thymus derived (T) cells by mitogens. II. Monocyte-macrophage potentiation of mitogen induced DNA synthesis. J. Immunol., 116:357-362, 1976. J.R. Schmidtke and S.H. Hatfield. Activation of purified human thymus derived (T) cells by mitogens: Diminished mitogen induced DNA synthesis in human T cells compared to autologous peripherabl blood lymphocytes. In press, Infection and Immunity, 13: , 1976.